There have been previous discussions of this. Since it is porcelain over cast iron, it will never have the tightness of a stainless steel top so it will allow steam to escape, especially at a boil. I sold LC for years, it is not steam tight.

Next time you cook try rotating the lid to get a better fit. Staub cast iron pieces have little nubs on the lid to let some steam out. Without any steam coming out you would have more of a pressure cooker. LC's lids are not airtight and some steam should come out.